(Patent Document 1)
Japanese Unexamined Patent Application Publication No. 11-130594
(Patent Document 2)
Japanese Unexamined Patent Application Publication No. 10-265293
(Patent Document 3)
Japanese Unexamined Patent Application Publication No. 8-259375
(Non-Patent Document 1)
Handbook of Applied Physics, 2nd edition, Maruzen, p. 427
With the increased high density integration of semiconductor elements, light sources employed in the various aspects of semiconductor fabrication come to use a beam having an ever shorter wavelength. Demand for a beam with a shorter wavelength currently reaches as far as the realm of vacuum ultraviolet rays. For the optical materials handling a beam with such a short wavelength, fluoride crystals have been used because of their high permeability to light. For example, for the optical materials used in an optical lithography apparatus to emit an ArF excimer laser (193 nm) or an F2 excimer laser (157 nm), single crystals of fluoride such as those of calcium fluoride, barium fluoride and magnesium fluoride have been used. Besides, development of new crystals of fluorides is urgently demanded in connection with the generation of ultraviolet and infrared lasers, UV cut windowpanes, optical materials for medical use, etc.
For the production of fluoride crystals, the Bridgman or CZ method has been principally employed to produce single bulk crystals. This mass of a single bulk crystal is cut into pieces according to given applications and measurement purposes (see, for example, Patent Document 1). Acquisition of single crystals by these methods, however, requires enormous cost and takes several days which significantly retards the efforts for the development of new fluoride materials. This is particularly true for the CZ method. When a fluoride crystal is grown by this method, a seed crystal is brought into contact with the top of fluoride melt. If some impurities float on the top, they must be removed. Such impurities, if any, may lengthen further the time of crystal growth and affect the quality of crystal.
On the other hand, for the production of single crystals of oxides and their eutectic mixtures, and of Si crystals, micro-pulling-down method has been practiced (Patent Documents 2 and 3, and Non-Patent Document 1). For example, Patent Document 2 describes a concrete apparatus based on the method in paragraph [0025] and FIG. 1.
By using the techniques disclosed in Patent Documents 2 and 3, and Non-Patent Document 2, it is possible to grow a single crystal at a pace significantly higher (in the order of 1 to 2) than conventional techniques. These techniques enable the contraction of time necessary for the growth of single crystals, and production of high quality crystals having a significantly large size from a more limited amount of material, as compared with conventional techniques. Furthermore, according to these techniques, crystal is pulled up through a tiny perforation formed at the bottom of a crucible, which eliminates the need for the removal of impurities floating on the surface of melt.
However, in the description of Patent Document 2, the technique is exclusively used for the production of single crystals of strong dielectric compounds such as LiNbO3, LiTaO3, KLN, etc. Also the technique described in Patent Document 3 is exclusively used for the production of single crystals of tungsten bronze structures represented by KLN, KLTN [K3Li2−2x(TayNb1−y)5+xO15+x], Ba1−xSrxNb2O6, etc., Mn—Zn ferrite, LiNbO3, YAG substituted by Nd, Er or Yb, YVO4 substituted by Nd, Er or Yb. Both the documents do not give any mention about the production of single crystals of fluorides.
The present invention aims to provide an apparatus enabling one to produce high quality crystals of fluorides in a very short period of time, method for producing such crystals, and a crucible suitably used for the apparatus.